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Date: Sat, 10 Jun 2017 17:48:05 -0400
From: David Edelsohn <dje.gcc@...il.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] s390x: Add single instruction math functions
On Sat, Jun 10, 2017 at 5:44 PM, David Edelsohn <dje.gcc@...il.com> wrote:
> On Sat, Jun 10, 2017 at 5:28 PM, Szabolcs Nagy <nsz@...t70.net> wrote:
>> * David Edelsohn <dje.gcc@...il.com> [2017-06-10 16:22:24 -0400]:
>>> On Sat, Jun 10, 2017 at 3:48 PM, Rich Felker <dalias@...c.org> wrote:
>>> > On Sat, Jun 10, 2017 at 02:53:14PM -0400, David Edelsohn wrote:
>>> >> On Sat, Jun 10, 2017 at 2:29 PM, Szabolcs Nagy <nsz@...t70.net> wrote:
>>> >> > * David Edelsohn <dje.gcc@...il.com> [2017-06-10 13:25:00 -0400]:
>>> >> >> On Sat, Jun 10, 2017 at 11:36 AM, Szabolcs Nagy <nsz@...t70.net> wrote:
>>> >> >> > * David Edelsohn <dje.gcc@...il.com> [2017-06-09 10:51:25 -0400]:
>>> >> >> >> The following patch is a start at single instruction math functions
>>> >> >> >> for s390x architecture to increase performance.
>>> >> >> >
>>> >> >> > looks good, i wonder why gcc does not have builtins support for
>>> >> >> > ceil, floor, nearbyint, round and trunc
>>> >> >> >
>>> >> >> > (on aarch64 the builtins expand to single instruction with
>>> >> >> > -fno-math-errno, but on s390x they remain libc calls
>>> >> >>
>>> >> >> Both the functions and builtins are converted to single instructions
>>> >> >> for me. What architecture level is your GCC assuming?
>>> >> >>
>>> >> >
>>> >> > i think it's the default s390x config
>>> >> >
>>> >> > $ s390x-linux-musl-gcc -v
>>> >> > Using built-in specs.
>>> >> > COLLECT_GCC=s390x-linux-musl-gcc
>>> >> > COLLECT_LTO_WRAPPER=/home/nsz/w/mcm/output/bin/../libexec/gcc/s390x-linux-musl/6.3.0/lto-wrapper
>>> >> > Target: s390x-linux-musl
>>> >> > Configured with: ../src_toolchain/configure --enable-languages=c,c++ CFLAGS='-g0 -Os' CXXFLAGS='-g0 -Os' LDFLAGS=-s --disable-nls --with-debug-prefix-map=/home/nsz/w/mcm/build-s390x-linux-musl= --enable-languages=c,c++ --disable-libquadmath --disable-libquadmath-support --disable-decimal-float --disable-multilib --disable-libcilkrts --disable-libvtv --disable-libgomp --disable-libitm --disable-werror --target=s390x-linux-musl --prefix= --libdir=/lib --disable-multilib --with-sysroot=/s390x-linux-musl --enable-tls --disable-libmudflap --disable-libsanitizer --disable-gnu-indirect-function --disable-libmpx --enable-libstdcxx-time --with-build-sysroot=/home/nsz/w/mcm/build-s390x-linux-musl/obj_sysroot
>>> >> > Thread model: posix
>>> >> > gcc version 6.3.0 (GCC)
>>> >> > $ cat a.c
>>> >> > double f(double x)
>>> >> > {
>>> >> > return __builtin_ceil(x);
>>> >> > }
>>> >> > $ s390x-linux-musl-gcc -O3 -fno-math-errno -S a.c -o -
>>> >> > .machinemode zarch
>>> >> > .machine "z900"
>>> >>
>>> >> Note the default architecture is z900 from 2005-2006. The FP
>>> >> instructions were added with the z196 processors in 2010.
>>> >
>>> > In that case the patch should probably have the code inside something
>>> > like:
>>> >
>>> > #ifdef __Z196__ // or whatever the predef macro for the ISA level is
>>> > // your code here
>>> > #else
>>> > #include "../foo.c"
>>> > #else
>>> >
>>> > See src/math/arm/sqrt.c for a similar example.
>>> >
>>> >> s390x-linux-musl probably should default to a much newer processor
>>> >> level, such as at least z196 or zEC12
>>> >
>>> > musl's policy is to just follow whatever ISA level the compiler is
>>> > configured for; you can set this at musl build time with CFLAGS or use
>>> > a default built into the toolchain at toolchain build time
>>> > (--with-arch, I think).
>>>
>>> Musl already defaults to the later ISA in the rest of the s390x port.
>>
>> would it be hard to support all s390x isa levels?
>
> It's a waste of effort and will hurt performance on newer processors.
>
> No user of Musl and Alpine is going to -- or even /can/ -- run it on
> older processors. All of the Docker containers and underlying Linux
> distributions don't support the older processors.
When I worked with Bobby Bingham to create the s390x port of Musl, I
said that he could assume newer processors. Also, I don't believe
that LLVM supports the earlier processors. I believe that he assumed
some more recent instructions in other parts of the code.
Thanks, David
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